1. Field of the Invention
This invention relates to an accumulator fuel injection system for diesel engines.
2. Description of the Prior Art
The conventional accumulator fuel injection systems for diesel engines include a fuel injection system disclosed in FIG. 10. This accumulator fuel injection system 60 consists of a fuel injection nozzle for supplying a fuel to be injected to an accumulator 5 so as to inject the fuel via an injection port 11, which is formed in a fuel injection nozzle body 15, into a combustion chamber in an engine, and a needle valve 3 for opening and closing the injection port 11 is provided in the accumulator 5 formed in the fuel injection nozzle body 15, a check valve 2 being provided slidably on this needle valve 3. This check valve 2 is formed so that it can utilize the needle valve 3 as a shaft thereof and move slidingly in the vertical direction along the outer circumferential surface thereof. The check valve 2 is urged by a spring 4 against a check valve seat 6 provided on the upper surface of the accumulator 5, whereby the communication between a fuel passage 13 and accumulator 5 is cut off, a clearance 7 being formed between the outer circumferential surface of the check valve 2 and the inner circumferential surface of the accumulator 5. In such a fuel injection system 60, the fuel sent under pressure by a fuel injection pump 8, which is driven by the engine, overcomes the spring 4 owing to the fuel supply pressure and enters the accumulator 5 through the check valve 2, injection pipe 9, fuel passage 13, through bore 14 in the check valve seat 6, and clearance 7 on the outer side of the outer circumferential surface of the check valve 2. When a period of the pressure-sending of the fuel by the fuel injection pump 8 has then finished, the pressure in the injection pipe 9, fuel passage 13 and through bore 14 in the check valve seat 6 suddenly decreases. Consequently, the force working on the upper surfaces of the needle valve 3 and check valve 2, i.e. a portion designated by a reference letter A is lost. As a result, the fuel pressure confined in the accumulator 5 overcomes the force of the spring 4 to slidingly move the check valve 2 and needle valve 3 in the upward direction. Accordingly, the injection port 11 is opened, and the fuel starts being ejected therefrom. When the injection of the fuel is started, the pressure in the accumulator 5 suddenly decreases, and the injection of the fuel into the combustion chamber is carried out until the force of the spring 4 has overcome the fuel pressure in the accumulator 5. A reference numeral 12 in the drawing denotes a slide surface of the needle valve 3.
Regarding this accumulator fuel injection system 60, a crank angle .theta., an injection rate R and a pressure P in the accumulator 5 have the relation shown in the graph of FIG. 11. The crank angle .theta. of the engine is plotted on the lateral axis, and the injection rate R and pressure P in the accumulator 5 on the longitudinal axis. In this case, the injection quantity is expressed by the equation: EQU Q=V/K.multidot.(Pi-Ps)
wherein V is the capacity of the accumulator; K the elastic modulus of the fuel; Pi the pressure in the accumulator; and Ps the pressure, which is determined by the force of the spring 4, for closing the needle valve 3. A curve L represents an injection quantity in a case of a low load, and a curve H an injection quantity in a case of a high load. A point B represents a point in time at which the pressure-sending of the fuel from the fuel injection pump 8 is started, and a point E a point in time at which the period of pressure-sending of the fuel from the fuel injection pump 8 is finished. A point D represents a point in time at which the injection of the fuel from the injection port 11 of the fuel injection nozzle in a case of a low load is started, and a point C or E a point in time at which the injection of the fuel from the injection port 11 in a case of a high load is started. A region from a point D to a point .theta..sub.4 represents a fuel injection period in a case of a low load, and a region from a point C to a point .theta..sub.5 a fuel injection period in a case of a high load. Accordingly, the fuel injection quantity Q in a case of a low load is shown by a curve l, and that in a case of a high load by a curve m.
The conventional fuel injection systems used for diesel engines further include a fuel injection system disclosed in Japanese Utility Model Laid-open No. 66164/1983. In this fuel injection system, a shaft having a needle valve for opening and closing an injection port is provided in an accumulator in an injection nozzle body which is provided with the injection port, and a check valve on this shaft slidably. The check valve is urged by a spring toward the upper surface of the accumulator to cut off the communication between a fuel injection passage and accumulator, and a clearance is provided between the outer circumferential surface of the check valve and the inner circumferential surface of the accumulator. A push rod is urged by a spring against the upper end surface of the shaft in the injection nozzle body to retain the push rod in a position in a clearance formed between the push rod and shaft, and the fuel injection passage is opened at the portion of the upper surface of the accumulator which is not in alignment with the upper surface of the shaft.
There is also a fuel injection nozzle disclosed in Japanese Utility Model Laid-open No. 133172/1985. In this fuel injection nozzle, a fuel from a pump is supplied from a fuel supply passage to a high-pressure chamber via a check valve and from the high-pressure chamber to a needle valve-holding accumulator via a check valve. The fuel supplied to the high-pressure chamber is pressurized by a smaller-diameter high-pressure plunger, which is formed integrally with a larger-diameter low-pressure piston, by applying a pressure to this piston, to accumulate the pressure in the accumulator, and the pressure applied to the low-pressure piston is then reduced to lift the needle valve, by which the injection port at the free end of the nozzle has been closed, and eject the fuel from the same injection port. This fuel injection nozzle is also provided with a pressure regulating means for lifting the needle valve in a stepped manner when the fuel is ejected.
A heat insulating engine in which a cylinder head, a cylinder liner, a piston head and a valve are formed out of a ceramic material has already been disclosed. In this heat insulating engine, the temperature at the end of a compression stroke thereof becomes extremely high in comparison with that in a regular engine, so that the time between the starting of the ejection of the fuel and the occurrence of ignition thereof, i.e. the ignition delay decreases to a great extent. This means that the quantity of the fuel injected into the combustion chamber until the engine ignition time decreases. Therefore, the premixed combustion immediately after the engine ignition decreases, and the performance of the engine is affected adversely and greatly. In order to eliminate these inconveniences, the fuel injection time is reduced to as great an extent as possible, and a required quantity of fuel is injected within this injection time, in a heat insulating engine. Accordingly, it is effective to use a unit injector capable of injecting a fuel under a high pressure Especially, in a heat insulating engine, an accumulator fuel injection system having a high initial injection rate at the injection starting time is optimumly used. If the accumulator fuel injection system 60 is applied to a heat insulating engine, in which an injection quantity Q is expressed by the equation, EQU Q=V/K.multidot.(Pi-Ps)
increasing Pi is one good method of increasing the injection rate R. However, if Pi is increased, it results in an increase in the maximum injection quantity assuming that Ps is constant. In order to prevent this from occurring, it is necessary to reduce the capacity V of the accumulator 5. Reducing the capacity V of the accumulator 5 is very difficult in view of the construction of the fuel injection nozzle.
In order to solve these problems, the inventor of the present invention developed an accumulator fuel injection system provided with a leak means in an accumulator, and filed previously Japanese Patent Application No. 187689/1987 (refer to Japanese Patent Laid-open No. 32063/1989) for the invention. The reliability of the operation accuracy of a control valve constituting the leak means in the accumulator fuel injection system is low in view of the construction thereof, and this leak means causes the pressure in the accumulator to decrease excessively when a leak occurs.
In the above-described fuel injection nozzle disclosed in Japanese Utility Model Laid-open No. 133172, a pressure regulating means for lifting the needle valve in a stepped manner is provided for the purpose of increasing the fuel pressure, atomizing the fuel, increasing the fuel particle scattering distance and reducing the injection time. However, since a high pressure is generated by the fuel injection pump so as to increase the pressure, a sufficiently high injection pressure cannot be obtained by this method. The effect of this method is limited due to the Hertz's stress in a cam member, and, moreover, the method causes the construction of the injection nozzle to become complicated. The fuel injection system disclosed in Japanese Utility Model Laid-open No. 66164/1983 referred to above is adapted to suppress an increase, which is ascribed to the combustion of a gaseous mixture, of the pressure in the combustion chamber when an engine load is high, by having the resultant force of springs act on, by increasing the injection pressure with a predetermined time lag, delaying an injection peak, and preventing the fuel from being injected at a suddenly increased pressure. However, this fuel injection system is not capable of solving the above-mentioned problems.